1. Field of the Invention
This invention relates to a method and apparatus for distributing wound yarn on a forming bobbin driven by a grooved drive roller in a collection station for textile yarns, a bobbin and a roller having shapes which are different from each other. In other words, this invention relates to a method and apparatus for distributing wound yarn on a conical bobbin driven by a grooved cylindrical roller, or for distributing wound yarn on a cylindrical bobbin driven by a grooved cone.
More specifically, said apparatus comprises a control unit based on a mini-computer into which the operative winding data are fed for processing and comparing with the data arriving from transducer probes or similar means, to provide at the minicomputer output a number of command signals for activating and controlling the motive source which angularly positions the bobbin carrier arm so that the collection station operates in regions not subject to the ribbing effect.
In the ensuing description and claims, the term "yarn" or "filament" indicates any type of filiform material, and the term "bobbin" indicates any package formed from yarn wound in substantially helical turns.
2. Discussion of the Background
In the state of the art a winding operation is used to form the yarn into a package suitable for subsequent processing. Winding by dragging is known, in which the bobbin is driven by friction by a grooved roller driven by the drive shaft. According to the above alternative cases, this roller can be a cylindrical roller with the produced bobbin being conical, or this roller can be a conical or frusto-conical element with the produced bobbin being cylindrical.
Within the helically arranged grooves the yarn is compelled to follow their movement, so that the rotation of the roller produces a corresponding rectilinear transnational movement of the yarn.
With reference to this field of the art, the problems strictly related to the principle of distribution of the yarn on the forming bobbin are of considerable importance. The winding units or stations arranged to produce bobbins of wound yarn almost always form deposits of turns concentrated at certain points, to form ribbing. This yarn distribution defect arises during bobbin winding when a wound turn is superposed on and is parallel to the previous turns.
This phenomenon occurs if the bobbin circumference is equal to the length of the winding turn, or to a multiple or a sub-multiple thereof. This type of winding in the form of substantially superposed turns results in hard bands within the bobbin. Said ribbing is known hereinafter as ribbing, taping or "mirror effects", these terms being used interchangeably.
These ribbing defects arise during winding if the ratio of the number of revolutions of the bobbin to the number of double beats of the traverser device or the number of complete to-and-fro movements of the yarn per unit of time is a whole number.
Under these conditions, after a complete to-and-fro movement of the yarn the starting point of the turns forming the new layer coincides with that of the previous layer.
This results in hardened superposed yarn layers forming ribbing, i.e. taping of maximum density, which compromises correct subsequent unwinding of the yarn or compromises the uniformity of liquid passage through bobbins during dyeing, resulting in layers not uniformly dyed and thus, leading to periodic variations in yarn color. To avoid these drawbacks a fractional ratio must be chosen so that the turns are advantageously displaced slightly from the corresponding underlying turn. It will be assumed that the bobbin rotational speed varies with time by virtue of maintaining its peripheral speed constant as its diameter grows, whereas the grooved drive roller, against which the bobbin rests, rotates at a constant speed with the result that the complete to-and-fro movements of the yarn remain constant with time.
Its ratio to the bobbin angular speed Wr varies continuously from a minimum (bobbin commencement) to a maximum (bobbin full), passing through intermediate whole-number values or exact fractional values. Said ratio is defined hereinafter as "winding ratio K" of the forming bobbin.
For each of said whole-number values or exact fractional values ribbing forms, i.e. the superposing of several yarn turns to give rise to the mirror effect. The aim of any winding system is therefore to deposit layers which do not generate problems during the subsequent use of the bobbin. For example the density of each layer must not differ from that of the preceding layer by more than a certain amount. Known methods and devices operate in various ways.
The most widely used methods involve discontinuity in the rotation of the bobbin under formation by raising it periodically from its driving contact with the underlying grooved cylindrical roller. The bobbin, which continues to rotate, gradually slows down until it is again brought into contact with the drive roller. It also forms part of the known art to periodically interrupt the rotation of the roller by cutting power to its electric drive motor, or by disengaging the grooved roller from the motor by a clutch or the like.
In recent years it has been proposed to periodically vary the speed of the motor which rotates the grooved roller. This method has considerable operational drawbacks because, as is well known to the expert of the art, its effect varies with varying the diameter of the bobbin under formation. The present applicant is the proprietor of the patent IT 1198214 relating to variation in the transmission of motion between the grooved drive roller and the bobbin for the purpose of varying their point of driving contact by virtue of the taper of the bobbin and/or of its deformability, hence varying the diameter on which it is driven and consequently their transmission ratio.
A different method proposed to prevent wound yarn turns from becoming superposed is described in DE 3521152.0 which describes and claims a method and device which effect a controlled and progressive variation in the rotational speed of the grooved drive element with simultaneous controlled braking of the bobbin, so that its peripheral speed remains constant within tolerance limits.